1. Field of the Invention
The subject invention generally pertains to mobile service rigs for wells and more specifically to a mobile service rig that includes an engine powering a hoist and a tong.
2. Description of Related Art
Oil wells and wells for other fluids typically include a well casing, tubing, sucker rods and a reciprocating drive unit. A well casing is what lines the well bore and usually comprises a long string of relatively large diameter pipe interconnected by threaded couplings known as collars. Casings generally define the overall diameter and depth of a well bore. Well tubing typically comprises a long string of pipe sections whose threaded ends are also interconnected by threaded couplings. The tubing extends down through the casing and provides a conduit for conveying oil or some other fluid to the surface of the well. A submerged reciprocating pump attached to the lower end of the tubing draws the fluid from the annulus between the inside diameter of the casing and the outside diameter of the tubing, and forces the fluid up through the tubing to the surface. To operate the pump, a string of sucker rods extends through the tubing to serve as a long reciprocating connecting rod that couples the submerged pump to a reciprocating drive unit at ground level. A string of sucker rods typically includes numerous sucker rods whose ends are interconnected by a threaded rod coupling.
Servicing oil wells and other types of wells can involve a variety of tasks that include, but are not limited to, installing or removing sections of casing, sucker rods, tubing and pumps. The various tasks each have their own particular needs.
When working with sucker rods, a rod tong is often used for making-up and/or disassembling a string of rods. A typical rod tong is a hydraulically powered wrench that turns one sucker rod relative to an adjacent one so that one or the other screws into or unscrews from the rods' adjoining coupling. Since sucker rods are continuously subjected to a pulsating or reciprocating load, fatigue may cause a rod coupling to separate if the coupling had been over or under tightened when it was first installed. Thus, sucker rods should be tightened in a precise manner.
The assembly of tubing is less critical, as tubing is generally stationary in a well bore. To assembly or disassemble tubing, a tubing tong is often used, which also is a hydraulically powered wrench. Tubing tongs have serrated teeth that grip the outer wall of two adjacent tube sections, and then tighten the two sections into their mating coupling. The operation typically involves substantially more power than what is required when working with sucker rods, as the diameter of tubing is significantly larger than that of rods.
Removing or replacing sections of casing often involves heavy lifting by way of a hoist operating at fill capacity. Full-power lifting may be required when the casing is stuck and difficult to remove from the well bore, or may be required simply due to the casing being relatively heavy. The hoist is also needed, but at a much lower lifting capacity, when installing or removing sucker rods. For tubing or for setting a pump, the hoist is generally operated at some intermediate capacity between that used for casings and sucker rods.
Since there are numerous tasks involved in servicing a particular well, and various wells can be hundreds of miles apart, it would be advantageous to equip a single vehicle with the all equipment needed to perform the various tasks. It would be further advantageous to provide such a vehicle with a single engine or prime mover to power the various equipment. However, that can be difficult to do, as the power requirements vary broadly among the various operations.
For example, to power or propel such a vehicle down the highway or to operate its hoist at full capacity may require a 400 hp diesel engine, while tightening or loosening sucker rods may only require 10 hp. Tightening or loosening tubing may require 30 hp. Thus if a single hydraulic pump is used to power both tubing tongs and rod tongs, such a pump should be able to provide 30 hp for tubing even though only 10 hp would be needed for tongs. Likewise, a single diesel engine should be able to provide 400 hp for vehicular transport and heavy hoisting even though only 30 hp is needed to power the hydraulic pump. The resulting power imbalances of such a system create some serious problems, particularly when installing or removing sucker rods.
With sucker rods, the rod tong typically operates at something less than 30 hp, while the hoist operates at a relatively low capacity (e.g., low weight, fast speed) to quickly move the sucker rods into position. The rod tong can preferably tighten or loosen a sucker rod coupling within the time it takes the hoist to get another rod into position. Thus, the hoist and the rod tong work in concert in removing or installing a string of sucker rods. To keep such an operation moving smoothly, an operator preferably does not divide his attention between the operations of the hoist and speed of the diesel engine (which powers the hoist and the pump that powers the rod tong). Thus, the operator typically just runs the engine at full speed, with the hoist transmission in low gear to keep the hoist operating at a reasonable speed. This wastes fuel, may tend to shorten the life of the engine, and generates a tremendous amount of waste heat in the hydraulic system that drives the rod tong.